This invention relates to a device for coupling a co-axial line used for transmitting a high frequency signal to another coaxial line and, especially, to a coupling device which enables relative rotation of both coaxial lines about their longitudinal axis without mutual entanglement.
For receiving satellite communication or satellite broadcast on a moving body such as vehicle or vessel, it is necessary to carry a microstrip or parabolic receiving antenna on the moving body and to direct it always to the satellite. Accordingly, the receiving antenna rotates with respect to the moving body with turning movement of the moving body and this may result in twist and entanglement of a coaxial cable connecting a convertor fixed to the antenna with a tuner fixed to the moving body. If the co-axial cable is elongated in order to suppress such twist and entanglement, it may wind round an antenna driving device and its attachments. It has been a general practice for avoiding this problem to cut the coaxial cable into two segments and insert a rotary joint therebetween.
The most primitive one of the rotary joints, as shown in the Japanese patent opening gazette No. 60169902, includes a pair of shells which are coupled to enable relative rotation along with their mutual contact and also electrically connected to the braids of outer conductors of two coaxial cables, respectively, a male pin which is insulatedly fixed to one of the shells and electrically connected to the central conductor or core of one of the coaxial cables, and a female pin which is insulatedly fixed to the outer shell and electrically connected to the central conductor or core of the other coaxial cable, and the male pin is inserted in the female pin so that they can relatively rotate in this state together with the shells. In such a coupling, however, the contact between the male and female pins is incomplete and a stray capacitance is formed therebetween. This stray capacitance, together with the contact resistance, varies with rotation and results in variable loses at the junction. Use of a spring or the like for improving the contact complicates the structure, and the mechanical contact lacks durability due to abrasion.
It has been proposed to capacitively couple both central conductors without the mechanical contact which is the cause of the above mentioned problems. In this case, circular discs are fixed normally to the tops of both central conductors and both discs are spaced at a fixed interval to form a capacitor. If the diameter of the discs is 10 mm and the interval is 1 mm, for example, the capacitance of this capacitor is about 1.5 pF. In case of transmitting a signal having a frequency of about 1 GHz, however, this results in a large impedance and reduced transmission loss characteristic as shown by curve A in FIG. 1. If a lumped constant coil 8 is inserted between each central conductor 2 and disc 6 as shown in FIG. 2 in order to cancel the capacitance between both discs, a stray capacitance is induced between the coil 8 and the shell 4 connected to the outer conductor as shown in phantom and the transmission loss characteristic is substantially improved as shown by curve B in FIG. 1. However, removal of discs 6 also has been considered, it would reduce excessively the distribution capacitance formed between both lumped constant coils 8, resulting, therefore, high Q which significantly reduces the bandwidth having low transmission loss as shown by curve C in FIG. 1.
Accordingly, an object of this invention is to provide a rotatable high frequency coaxial line coupling device which exhibits a low transmission loss over a relatively wide bandwidth.